Colossal pressure-induced softening in scandium fluoride

The counter-intuitive phenomenon of pressure-induced softening in materials is likely to be caused by the same dynamical behaviour that produces negative thermal expansion. Through a combination of molecular dynamics simulation on an idealised model and neutron diffraction at variable temperature and pressure, we show the existence of extraordinary and unprecedented pressure-induced softening in the negative thermal expansion material scandium fluoride, ScF${}_3$, with values of the pressure-derivative of the bulk modulus $B$, $B^{\mathrm{\prime }}=\mathrm{\partial }B\mathrm{/}\mathrm{\partial }P$, reaching as low as $-40\pm 1$.